Up to now, an optical disc enabling information signals to be recorded and/or reproduced using an optical beam has been used. Among the optical discs of this sort in use, there are a read-only compact disc (CD) having pre-recorded music data or information data to be processed by a computer, and a CD-ROM.
There have also been proposed a high-density (HD) disc or a high density (HD)-CD-ROM having the recording density twice or three times as high as that of the CD or CD-ROM having the standard recoding density.
There is also proposed a digital versatile disc/digital video disc (DVD) as an optical disc having the same size as but having the recording density seven to eight times as high as the CD or the CD-ROM.
Also in use is a magneto-optical disc which is a recording/reproducing optical disc having the same size as the CD or CD-ROM and capable of re-recording data signals.
An optical disc drive device, having the above optical discs as a recording medium, has a disc table which is run in rotation by a spindle motor and in which the optical disc is set in a horizontal position. The signal recording surface of the optical disc, thus rotated, is scanned across its inner and outer rims by a light beam radiated from an optical pickup for recording/reproducing data on or from the optical disc. The light beam radiated from the optical pickup is radiated perpendicularly on the signal recording surface of the optical disc as it is converged for being focused on the signal recording surface.
For recording and/or reproducing data on or from the optical disc with satisfactory recording/reproducing characteristics, it is desirable that the light beam radiated from the optical pickup be incident perpendicularly on the signal recording surface of the optical disc. With the light beam being incident perpendicularly on the signal recording surface of the optical disc, a beam spot of the light beam converged by the objective lens for being radiated on the signal recording surface of the optical disc becomes truly circular, as a result of which the beam spot can correctly scan a recording track formed on the signal recording surface of the optical disc for recording and/or reproducing data with good recording/reproducing characteristics.
Meanwhile, in consideration of warping of an optical disc and mounting accuracy of a spindle motor or an optical pickup, a pre-set allowance is provided in an optical disc driving device employing an optical disc as a recording medium by a standard with respect to a relative tilt between the signal recording surface of the optical disc and the optical axis of the objective lens converging and radiating the light beam on the optical disc.
For example, in an optical disc driving device employing an optical disc, such as a CD or CD-ROM 12 cm in diameter, as a recording medium, the allowance of the relative tilt between the optical axes of the optical disc and the objective lens is unified at 1.2.degree. or less. That is, the relative tilt of 1.2.degree. or less between the optical axis of the optical disc and that of the objective lens with respect to the optical axis of the objective lens perpendicular to the signal recording surface of the optical disc is allowed. Specifically, the allowance of the tilt of the objective lens is set to 0.6.degree. or less, while that of the spindle motor or of the objective lens on the device is set to 0.6.degree. or less.
On the other hand, with an optical disc, such as HD-CD or HD-CD-ROM, having a recording density higher than that of a CD or a CD-ROM having a standard recording density, not only the recording track for data recording is reduced in width, but also the track pitch is reduced for increasing the recording density per unit area for improving the recording density.
In an optical disc drive device employing the optical disc of such high recording density as a recording medium, it is necessary to set verticality of the objective lens with respect to the optical disc to a higher accuracy. If, when the optical disc of high recording density is used as a recording medium, the verticality of the objective lens with respect to the optical disc is not maintained at high accuracy, the beam spot of the light beam radiated on the signal recording surface is distorted in shape and narrow in width such that the recording track of narrow pitch width cannot be scanned correctly. For example, it may be an occurrence that a sole beam spot radiates the neighboring recording tracks simultaneously such that the light beam radiated on a pre-set recording track is reduced in energy density with the result that data recording/reproduction cannot be performed correctly.
This it is not possible to record and/or reproduce data on or from an optical disc of high recording density with the use of an optical disc player employing a CD or a CD-ROM having a standard recording density.